Distillate sweetening process



Patented Apr. 3, 1951 J U TE sT rEs PATENT OFFICE I v i Q Q I IDISTILLATE raocss I Ind., assignors to Standard Oil Company, Ohicago,111., a corporation of Indiana No Drawing. Application January 12, 1949,Serial No. 70,584

In the manufacture of heater oils from high sulfur crudes, it has beenfound that, whereas the sour heater oil distillate gives satisfactoryperformance in burning tests, the same oil gives objec'tionable depositsin certain sleeve-type burn- Claims. (01.196-32) 2 or preferably oxygeninto the oil as it leaves the last stage of the caustic-methanolextraction and then allow the oil which is saturated with oxygen tostand for a short time before finally Washing with water to remove thelast traces of caustic and dissolved methanol.

The reaction is quite rapid and'usually requires onlyabout one-half hourat room temperature, e. g. 60 to 80 F. In winter when the oil is cold,it is desirable to warm it to a temperature within the range of about60to 100 F. in order to hasten 1 the reaction of the oxygen and mercaptansre ers after it has been subjected to sweetening with the usual reagentssuch as doctor solution, cope perchloride, etc. However, if the oil .isextracted with'a caustic-methanol solution which chemicall'y removes theill-smelling mercaptans instead of simply converting them to disulfides, then the oil .will give satisfactory performance in burner operation.

The removal of mercaptans bythe use of caustic-methanol suffers thedisadvantage,however, in that it is extremely difiicult to lower themercaptan number of the oil to a point where it will pass the usualdoctor test for a'sweet oil. In other words, the caustic-methanolsolution is not generally capable of making the oil salable and it iscommon practice to follow caustic-methanol treatment with doctorsweetening or copper sweetening. The caustic-methanol treatment appliedin stages readily reduces the mercaptan number to about '5 or but it issubstantially impossible to reduce the mercaptan numberbelow 1 which isordinarily accepted as the upper limit for a sweet oil. The mercaptannumber, sometimes called the copper number, is the milligrams ofmercaptan sulfur per 100 milliliters of oil, generally determined bytitration with a standardized copper salt solution.

An object of our invention is to eliminate the z extra treatment ofcaustic-methanol extracted oils with doctor solution, copper chloride,sodium hypochlorite, or other usual sweetening reagent. Another objectof our invention is to produce oils which will satisfactorily meet thedoctor. test when treated by the caustic-methanol process. Still anotherobject of our invention is to facilitate washing of oil treated withcaustic-methanol solutions.

We have discovered that the caustic-methanol treated oil can besatisfactorily sweetened by contacting it with oxygen or air after theseparation of the caustic-methanol solution and before washing withwater or neutralization of the caustic. For our purpose it is sufiicientto inject air maining in the oil after caustic-methanol extraction. Thetime required for the reaction is usually only about twenty minutes tothreehours andtherefore, in a continuous operation, it is not necessaryto maintain a large volume of the oil-in contact with oxygen.Accordingly, only a small aging tank may be required for the oxygentreat ment, after which the oilmay be washed with water to recoverdissolved methanol and then-sent immediately to storage or to themarket. The oxygen or air treatment also aids in washing the oil bright,the strong caustic-methanol solution having a tendency to leave acolloidal dispersion in the. oil which is difiicult to wash out withwater.

. Where the extraction of mercaptans by the caustic-methanol isespecially efficient and the mercaptan number of the extracted oil islow, e. g. 2 to 5, the oil can often be sweetened by the oxygentreatment by simply injecting oxygen into the line leading from thecaustic-methanol extractor to the final water-washing step of theprocess. Rapid solution of oxygen in the oil can be obtained byinjecting oxygen into the stream of oil just before entering acentrifugal pump where excellent mixing and diffusion of the oxygen intothe oil is obtained. If the oxygen is injected under pressure, e. g. 10to p. s. i., the rate of solution-is increased, thereby still furtherreducing the time required for the sweetening effect. The amount ofoxygen required is quite small and varies with the residual mercaptansin the extracted oil. Thus, with an oil having a mercaptan number ofabout 5, it is desirable to inject from about 5 pounds to about 20pounds of oxygen per thousand barrels.

As an example of our process, a sour West system.

her was reduced only to 7.5 with this treatment, showing that airblowingthe neutralized oil was inefiective in producing a marketable product.

Another oil having a mercaptan number of 63 was extracted with one-fifthvolume of a causticmethanol solution giving a mercaptan number of 8.5.After washing with water the oil was blown with air for one-half hour atroom temperature and then tested 6 mercaptan number.

When blown with air for one-half hour at room temperature before washingwith water, the mercaptan number was reduced from 8.5 to 1.5.

In another operation on caustic-methanol extracted oil, the mercaptannumber was reduced to 0.5 by the action of oxygen injected into the oilafter the extraction operation and before washing. This oil was sweet tothe doctor test,

4 that contains mercaptans which comprises extracting said oil with anaqueous caustic-methanol solution until the major part of the mercaptansare removed from the oil, separating substantially all the mercaptanrich solution from the oil, and contacting the extracted oil which stillcontains some caustic and methanol with a gas containing free oxygenuntil the oil is substantially mercaptan-free.

2. The process of claim 1 wherein the substantially mercaptan-free oilis washed with water to recover methanol.

3. The process of claim 1 where in the mercaptan number of the extractedoil is less than i. The processof claim 1 wherein the free oxygen.containing gas is injected into a flowing that is, a sample shaken witha solution of litharge in 10 per cent NaOI-I to which a pinch of sulfurwas added, gave no orange-yellow precipitate or rag.

The caustic-methanol solution employed in the extraction operation isusually prepared from a strong solution of NaOH or KOH containing from40 to 65 per cent caustic in water, the amount of methanol usually beingabout 10-to a volumes per hundred volumes of the aqueous causticsolution. If desired there may be also incorporated in the solutionalkali phenolates,

cresylates, etc. to increase the mercaptan extrac- 12.

desirable to pass the air, after leaving the oil, thru a water scrubberto recover vapors of. methanol which would otherwise be lost from theThe methanol dissolves in the water from which it can be recovered bydistillation for re-use' in the system, or the water solution ofmethanol can be used in the regeneration of spent-caustic-methanolsolution. Having thus described our invention, what w .claim is:

, .1. The process of sweetening a hydrocarbon oil 0 When air is used tocontact the oil, it is often stream of the extracted oil.

5. The process of claim 1 wherein the amount of free oxygen added isbetween about and 20 pounds per thousand barrels of extracted oil.

6. The process of sweetening a sour. hydrocare bon oil which boils abovethe gasoline range which comprises extracting said sour oil withv anaqueous caustic-methanol solution until the mercaptan number of theextracted oil is between about 2 and 10, separating substantially allthe mercaptan rich aqueous caustic-methanol solution, contacting theextracted oil which still con-.- tains some caustic and methanol forabout 20 to 180 minutes with from about to pounds ofiree oxygen perthousand barrels-of extracted oil, and washing the treated oil withwater to re: cover methanol therefrom. 7. The process of claim 6 whereinthe aqueous caustic-methanol solution has the following composition:from about 10 to volumes of meth: anol per volumes of aqueous solutionc011? taining 40-60 weight percent of caustic.

THEODORE B. TOM. JACK H. KRAUSE.

' REFERENCES crrEn Thefollowing references are of record in-the file ofthis patent:

UNITED STATES PATENTS 2,297,537. Craig et a1. Sept. 29, 1942

1. THE PROCESS OF SWEETENING A HYDROCARBON OIL THAT CONTAINS MERCAPTANSWHICH COMPRISES EXTRACTING SAID OIL WITH AN AQUEOUS CAUSTIC-METHANOLSOLUTION UNTIL THE MAJOR PART OF THE MERCAPTANS ARE REMOVED FROM THEOIL, SEPARATING SUBSTANTIALLY ALL THE MERCAPTAN RICH SOLUTION FROM THEOIL, AND CONTACTING THE EXTRACTED OIL WHICH STILL CONTAINS SOME CAUSTICAND METHANOL WITH A GAS CONTAINING FREE OXYGEN UNTIL THE OIL ISSUBSTANTIALLY MERCAPTAN-FREE.